1. Field of the Invention
This invention relates generally to so-called "carbonless copying paper." This is pressure-sensitive record sheet material of the type utilizing pressure-rupturable microcapsules of a solution of color-reactant dye precursor which produces a colored mark when placed in contact with pigment particles after a barrier normally separating the dye precursor and pigment particles is broken by printing pressure. More specifically, the instant invention relates to the use of such record material which utilizes encapsulated alkali-sensitive color precursors and alkaline pigments to develop the color of the dye precursor.
Pressure-sensitive record material based on the use of microencapsulated color precursors and reactive pigments is old in the art. Such record material is of two general types. In one the color precursor is an acid-sensitive material and it reacts with an acidic pigment or acidic resin to produce a colored species. An example is the record material which utilizes an encapsulated solution of crystal violet lactone mixed with benzoyl leucomethylene blue as the dye precursor and attapulgite clay or Japanese acid clay as the pigment. Another example of this type of record material employs crystal violet lactone and a phenolic resin mixed with kaolin clay. A second type of pressure-sensitive record material features the use of an encapsulated complexing agent which reacts with a metal-bearing coating to produce a colored metal complex.
Currently available pressure-sensitive record material leaves much to be desired. The pigments that are useful with acid-reacting dyes exemplified by crystal violet lactone are costly and the developed dyes tend to fade rapidly during use. Fading of the image is minimized somewhat by using a secondary dye precursor such as benzoyl leucomethylene blue. However, rapid fade of crystal violet lactone im ages is still a problem that awaits a practical solution. Furthermore, the pigments which are effective with acid-reacting dyes leave much to be desired in ease (hence cost) of coating. Print receiving sheets coated with acidic resin systems tend to yellow and develop an odor on storage. Further, the sheets do not produce printed images of strong intensity and the sheets tend to lose sensitivity on aging. On the other hand, record material based upon the use of a color precursor which is a complexing agent produces a printed image of weak intensity and undesirable hue.